Heat exchanger with a reduced tendency to produce deposits and method for producing same

ABSTRACT

The invention relates to a process for the production of a heat transfer device, which comprises electroless chemical deposition of a metal/polymer dispersion layer, in which the polymer is halogenated, on a heat transfer surface. The invention furthermore relates to a process for the production of a heat transfer device, wherein a metal/phosphorus layer with a thickness of from 1 to 15 μm is applied by electroless chemical deposition before application of the metal/polymer dispersion layer. The invention furthermore relates to a heat transfer device which can be produced by a process according to the invention, and to the use of a coating, produced by electroless chemical deposition of a metal/polymer dispersion layer, in which the polymer is halogenated, for reducing the tendency of the coated surfaces to accumulate solids from fluids, causing fouling.

The present invention relates to a process for the production of heattransfer devices which comprises electroless chemical deposition of ametal/polymer dispersion layer. The present invention furthermorerelates to heat transfer devices according to the invention. The presentinvention furthermore relates to the use of a metal/polymer dispersionlayer as permanent encrustation inhibitor.

In recent decades, all branches of industry have suffered from foulingin heat transfer devices (Steinhagen et al (1982), Problems and Costsdue to Heat Exchanger Fouling in New Zealand Industries, Heat TransferEng., 14(1), pages 19-30). When designing heat exchangers, increasingfrictional pressure loss and heat-transfer resistance due to foulingmust be taken into account. This results in over-dimensioning of heattransfer devices by from 10 to 200%.

The development of anti-fouling methods has therefore taken onconsiderable importance.

Mechanical solutions have the disadvantage of being restricted torelatively large heat exchangers and in addition of causing considerableincreased costs. Chemical additives can result in undesiredcontamination of the product and in some cases pollute the environment.For these reasons, ways of reducing the fouling tendency by modifyingthe heat-transfer surfaces have recently been sought. Although surfacecoatings with organic polymers, such as polytetrafluoroethylene (PTFE),reduce the fouling tendency, the known coatings themselves causesignificant additional heat transmission resistance. At the same time,durability reasons mean that the layer thickness has a lower limit.Similar problems are also observed in methods which involve applyingmonolayer silane coatings to the surface to be protected (Polym. Mater.Sci. and Engineering, Proceedings of the ACS Division of PolymericMaterials Science and Engineering (1990), Volume 62, pages 259 to 263).

The problems associated with the use of polymer coatings do not occur ina process described in WO 97/16692. In this process, the hydrophobicityof the surface is increased by ion implantation or by sputteringmethods. Although this results in a reduction in the fouling tendency,the use of this process, which always requires vacuum techniques, is,however, very expensive. In addition, the processes described are notsuitable for coating poorly accessible or complex-shaped surfaces orcomponents with a uniform layer.

The deposits whose formation is to be prevented are inorganic salts,such as calcium sulfate, barium sulfate, calcium carbonate and magnesiumcarbonate, inorganic phosphates, silicic acids and silicates, corrosionproducts, particulate deposits, for example sand (river and sea water),and organic deposits, such as bacteria, algae, proteins, mussles andmussle larvae, polymers, oils and resins, and biomineralized compositesconsisting of the above-mentioned substances.

It is an object of the present invention to indicate a process for theproduction of a heat transfer device which, on the one hand, reduces thetendency of the heat-transfer surfaces to accumulate deposits of solids,causing fouling, and which, on the other hand, results in negligibleheat transmission resistance while having high stability (for example toheat, corrosion and underwashing). At the same time, the surfacestreated by the process should have satisfactory durability. The processshould also be inexpensive to use on poorly accessible surfaces.

We have found that this object is achieved by a process for theproduction of a heat transfer device which comprises electrolesschemical deposition of a metal/polymer dispersion layer, in which thepolymer is halogenated, on a heat transfer surface.

For the purposes of the present invention, a heat transfer device is adevice which has surfaces designed for heat exchange (heat transfersurfaces). Preference is given to heat transfer devices which exchangeheat with fluids, in particular with liquids.

Heating elements and heat exchangers, in particular plate heatexchangers and spiral heat exchangers, are preferred embodiments of heattransfer devices.

A halogenated polymer is a fluorinated or chlorinated polymer;preference is given to fluorinated polymers, in particularperfluorinated polymers. Examples of perfluorinated polymers arepolytetrafluoroethylene (PTFE) and perfluoroalkoxy polymers (PFA, inaccordance with DIN 7728, Part 1, January 1988).

This solution according to the invention is based on a process forelectroless chemical deposition of metal/polymer dispersion phases whichis known per se (W. Riedel: Funktionelle Vernickelung [Functional NickelPlating], Eugen Leize publishers, Saulgau, 1989, pages 231 to 236, ISBN3-750480-044-x). A metal/polymer dispersion phase comprises a polymer,for the purposes of the present invention a halogenated polymer, whichis dispersed in a metal alloy. The metal alloy is preferably ametal/phosphorus alloy.

The processes employed hitherto for preventing the encrustation tendencyresulted in surfaces having greater roughness than electropolished steel(see Table 1). It is now been found that a coating which also reducesthe roughness does the same job. In addition, it has been found that theeffect of the polymer component in reducing the encrustation tendency iscrucial, although the polymer content in the dispersion layer is ratherlow, at from 5 to 30% by volume.

In addition, it has been found that the surfaces treated in accordancewith the invention facilitate good heat transfer, although the coatingscan have a not inconsiderable thickness of from 1 to 100 μm. Thesurfaces treated in accordance with the invention furthermore havesatisfactory durability, which also allows layer thicknesses of from 1to 100 μm to appear appropriate; the layer thickness is preferably from3 to 20 μm, in particular from 5 to 16 μm. The polymer content of thedispersion coating is from 5 to 30% by volume, preferably from 15 to 25%by volume, especially from 19 to 21% by volume. Furthermore, thecoatings used in accordance with the invention are, as a result theprocess, relatively inexpensive and can also be applied to poorlyaccessible surfaces. These surfaces can be any desired heat transfersurfaces, such as internal surfaces of pipes, surfaces of electricalheating elements and surfaces of plate heat exchangers, etc., which areused for heating or cooling fluids in industrial plants, in privatehouseholds, in food processing or in power generation or water treatmentplants.

“Heat transmission” means the transfer of heat from the interior of theheat transfer device to any coating present on the fluid side, heatconduction within the coating layer, and heat transfer from the coatinglayer to the fluid (for example a salt solution).

In a preferred embodiment of the process according to the invention, themetal/phosphorus alloy of the metal/polymer dispersion layer iscopper/phosphorus or nickel/phosphorus, preferably nickel/phosphorus.

In a further embodiment of the process according to the invention, thenickel/polymer dispersion layer is a dispersion layer ofnickel/phosphorus/polytetrafluoroethylene. However, other fluorinatedpolymers are also suitable, such as perfluoroalkoxy polymers (PFA,copolymers of tetrafluoroethylene and perfluoroalkoxy vinyl ethers, forexample perfluorovinyl propyl ether). If the heat transfer device is tobe operated at relatively low temperature, the use of chlorinatedpolymers is likewise feasible.

In contrast to electrodeposition, the electrons required for chemical orautocatalytic deposition of the nickel/phosphorus are not provided by anexternal power source, but instead are generated by chemical reaction inthe electrolyte itself (oxidation of a reducing agent). The coating iseffected by dipping the workpiece into a metal electrolyte solutionwhich has previously been mixed with a stabilized polymer dispersion.The dipping operation is preferably followed by conditioning at from 200to 400° C., in particular at from 315 to 325° C. The conditioningduration is generally from 5 minutes to 3 hours, preferably from 35 to45 minutes. Examples of metal solutions which can be employed arecommercially available nickel electrolyte solutions containing Ni^(II),hypophosphite, carboxylic acids and fluoride and, if desired, depositionmoderators, such as Pb²⁺. Such solutions are sold, for example, byRiedel, Galvano-und Filtertechnik GmbH, Halle, Westphalia, and AtotechDeutschland GmbH, Berlin. Polymers which can be used are, for example,commercially available polytetrafluoroethylene dispersions (PTFEdispersions). Preference is given to PTFE dispersions having a solidscontent of from 35 to 60% by weight and a mean particle diameter of from0.1 to 1 μm, in particular of from 0.1 to 0,3 μm, wherein the particleshave a spherical morphology, and which contain a neutral detergent (forexample polyglycols, alkylphenol ethoxylate or, if desired, mixtures ofthese substances, from 80 to 120 g of neutral detergent per liter) andan ionic detergent (for example alkyl- and haloalkylsulfonates,alkylbenzenesulfonates, alkylphenol ether sulfates, tetraalkylammoniumsalts or, if desired, mixtures of these substances, from 15 to 60 g ofionic detergent per liter). Typical dip baths have a pH of about 5 andcontain about 27 g/l of NiSO₄×6 H₂O and about 21 g/l of NaH₂PO₂×H₂O witha PTFE content of from 1 to 25 g/l. The polymer content of thedispersion coating is affected principally by the amount of polymerdispersion added and the choice of detergents.

The present invention furthermore relates to a process for theproduction of a heat transfer device which has a particularly adherent,durable and heat-resistant coating and therefore achieves the objectaccording to invention in a particular manner. This process is based ona process for the production of a heat transfer device which compriseselectroless chemical deposition of a metal/polymer dispersion coating,in which the polymer is halogenated, onto a heat transfer surface.

This process additionally comprises applying a metal/phosphorus layerwith a thickness of from 1 to 15 μm by electroless chemical depositionbefore application of the metal/polymer dispersion layer.

Electroless chemical deposition of a metal/phosphorus layer with athickness of from 1 to 15 μm for improving adhesion is carried out bymeans of the metal electrolyte baths described above, but to which inthis case no stabilized polymer dispersion is added. Conditioning ispreferably not carried out at this time, since this generally has anadverse effect on the adhesion of the subsequent metal/polymerdispersion layer. After deposition of the metal/phosphorus layer, theworkpiece is introduced into the dip bath described above, which,besides the metal electrolyte, also contains a stabilized polymerdispersion. The metal/polymer dispersion layer forms during thisoperation. This is preferably followed by conditioning at from 200 to400° C., in particular at from 315 to 325° C. The conditioning durationis generally from 5 minutes to 3 hours, preferably from 35 to 45minutes.

In a further embodiment of the process according to the invention, themetal/phosphorus layer has a thickness of from 1 to 5 μm.

In a further embodiment of the process according to the invention, themetal/phosphorus alloy of the metal/polymer dispersion layer and of themetal/phosphorus layer is nickel/phosphorus or copper/phosphorus.

In a further embodiment of the process according to the invention, themetal/polymer dispersion layer is a dispersion layer ofnickel/phosphorus/polytetrafluoroethylene.

The invention furthermore relates to a heat transfer device which can beproduced by a process according to the invention. The heat transferdevice according to the invention is preferably produced using a processaccording to the invention.

In a further embodiment, the above-mentioned heat transfer deviceaccording to invention is designed for the transfer of heat to fluids,in particular to liquids. Suitable heating elements here are all thosewhich transfer heat to fluids. Furthermore, heat exchangers, inparticular plate heat exchangers and spiral heat exchangers, arepreferred examples of such heat transfer devices.

The invention furthermore relates to the use of a coating produced byelectroless chemical deposition of a metal/polymer dispersion layer, inwhich the polymer is halogenated, for reducing the tendency of thecoated surfaces to accumulate solids from fluids, causing fouling. Thefluids are preferably liquids. The fouling whose formation is preventedin accordance with the invention has already been described.

Some advantages of the heat transfer devices according to the inventionor their coatings are indicated by the attached drawing, in which:

FIG. 1 shows the heat transfer coefficient through the boundary layer asa function of time, taking into account any coating layer present, oncontact of various heat exchanger surfaces with a boiling salt solution,and

FIG. 2 shows the heat transfer coefficient through the boundary layer asa function of time, taking into account any coating layer present, oncontact of various heat exchanger surfaces with a warm stream of saltsolution.

FIG. 1 shows the decrease in the heat transfer coefficient (α [W/m²K])due to CaSO₄ deposits as a function of time (t [min], abscissa) forvarious heat transfer devices which differ in the nature of theirsurfaces. Reference numeral 1 refers to the measured values of thecoating according to the invention from the Example (*7). Referencenumeral 2 denotes the measured values for an electropolished steelsurface. The power per unit area is 200 kW/m², the concentration of theCaSO₄ solution is 1.6 g/l and the temperature corresponds to the boilingpoint.

FIG. 2 shows the measured decrease in the heat transfer coefficient (α[W/m²K]) due to CaSO₄ deposits as a function of time (t [min], abscissa)for various heat transfer devices which differ in the nature of theirsurfaces. Reference numeral 1 refers to the coating according to theinvention from the Example (*7). Reference numeral 3 refers to anuntreated steel surface. The power per unit area of the heat transferdevice is 100 kW/m². A CaSO₄ solution having a concentration of 2.5 g/lflows past the heat transfer device at a velocity of 80 cm/s and atemperature of 80° C.

Example

The advantages of the heating surfaces coated in accordance with theinvention compared with uncoated heating surfaces, electropolishedsurfaces and ion-implanted or sputtered surfaces were determined inlaboratory investigations. Table 1 contains a comparison of the measuredvalues for surface roughness, surface energy and wetting angle of theheating surfaces investigated, and the relative decrease in the measuredheat transfer coefficients within the first 100 hours of the experiment.It is apparent that the heat transfer devices according to the inventionprovide very low surface energy, a very large contact angle and verygood heat transfer behavior.

TABLE 1 Surface Contact Rough- energy angle ness, α₁₀₀/α₀ [mJ/m²] * [°]** μm **** *** Untreated (steel) 84 65 0.14 0.4 Electropolished steel 8662 0.08 0.65 Si-ion implanted steel *5 39 80 0.14 0.75 F-ion implantedsteel *5 37 82 0.14 0.9 DLC-sputtered steel *6 36 85 0.13 0.85TiNF-sputtered steel *6 34 87 0.14 0.9 Steel/Ni-PTFE *7 25 100 0.1 0.9

Table 2 shows the surface energy, contact angle and bacteria(Streptococcus thermophilus) deposited per unit area of the heattransfer devices according to the invention compared with the heattransfer devices of the prior art.

TABLE 2 Surface Contact log10 energy angle cells/cm² [mJ/m²] * [°] ** *9Untreated (steel) 84 65 5.7 Electropolished steel 86 62 5.5 Si-ionimplanted steel *5 39 80 4.9 F-ion implanted steel *5 37 82 5.5DLC-sputtered steel *6 36 85 5.0 CrC-sputtered steel *6 34 87 4.1Steel/Ni-PTFE *7 25 100 3.9 *Measurement by the method of A. J. Kinloch,Adhesion and Adhesives, Chapman & Hall, University Press, Cambridge,1994 **Measurement by the method of K. K. Owens, J. of Appl. Polym. Sci.13 (1969) 1741-1747 ***Relative heat transfer coefficient after anoperating time of 100 hours (by the method of Müller-Steinhagen et al.,Heat Transfer Engineering 17 (1998), 46-63) ****Surface roughness, Ra inaccordance with DIN ISO 1302 *5 Method as described by J. W. Mayer, “IonImplantation in Semiconductors, Silicon and Germanium”, Academic Press,1970 (ISBN 75107563) *6 Process for the application of diamond-likecarbon DLC in accordance with GB-A 9006073 *7 Firstly, a chemicallyelectroless nickel layer of 5 μm containing 8% of phosphorus was appliedfor improving adhesion by immersion in a chemically electroless nickelelectrolyte solution. The Ni/phosphorus/ PTFE dispersion coating wassubsequently produced in a dip bath consisting of a mixture of achemically electroless nickel electrolyte solution and adetergent-stabilized PTFE dispersion. The deposition ofnickel/phosphorus/polytetrafluoroethylene was carried out at from 87 to89° C., i.e. at below 90° C., and at a pH of the electrolyte solution offrom 4.6 to 5.0. The deposition rate was 10 μm/h, and the layerthickness was 15 μm. The composition of the chemically electrolessnickel electrolyte/PTFE solution is shown in Table 3.

TABLE 3 Concentration [g/l] pH NiSO₄ × 6H₂O 27 4.8 NaH₂PO₂ × H₂O 21CH₃CHOHCOOH 20 C₂H_(5COOH) 3 Na citrate 5 NaF 1 PTFE (50%) 8* 2-50

Chemically electroless nickel electrolyte solutions are commerciallyavailable (Riedel, Galvano- und Filtertechnik GmbH, Halle, Westphalia,and Atotech Deutschland GmbH, Berlin). After application of thenickel/phosphorus/PTFE layer, the workpiece was conditioned at 300° C.for 20 minutes. The polymer and phosphorus contents in the dispersionlayer were 20% by volume of PTFE, corresponding to 6% by weight of PTFE,and 7% of phosphorus.

*8 The PTFE dispersions are commercially available. The solids contentand mean particle size were 50% by weight and 0.2 μm respectively. Thedispersion was stabilized by a neutral detergent (50 g/l of Lutensol®alkylphenol ethoxylate, 50 g/l of Emulan® alkylphenol ethoxylate,manufacturer of both detergents is BASF AG, Ludwigshafen) and an ionicdetergent (15 g/l of Lutensit® alkyl-sulfonate, BASF AG, Ludwigshafen, 8g/l of Zonyl® perfluoro-C₃-C₈-alkylsulfonate, Dupont, Wilmington, USA).The concentration FIGS. 2-50 g/l relates to the amount of dispersionsolution added.

*9 The measurement was carried out by the method of H.Muller-Steinhagen, Q. Zao and M. Reiβ, “A novel low fouling metal heattransfer surface”, 5th UK National Conference on Heat Transfer, London,Sep. 17-18, 1997. The cell culture is Streptococcus thermophilus.

We claim:
 1. A process for the production of a heat transfer device forexchange of heat with fluids, wherein a) a metal/phosphorus layer with athickness of from 1 to 5 μm is applied by electroless chemicaldeposition onto a heat transfer surface and b) a metal/polymerdispersion layer, win which the polymer is halogenated, is subsequentlyapplied by electroless chemical deposition onto the metal/phosphoruslayer, produced in step a), and said metal/polymer dispersion layer hasa polymer content of from 5 to 30% by volume.
 2. A process as claimed inclaim 1, wherein the metal/phosphorus alloy of the metal/polymerdispersion layer and of the metal/phosporous layer is nickel/phosphorusor copper/phosphorus.
 3. A process as claimed in claim 1, wherein themetal polymer/dispersion layer is a dispersion layer ofnickel/phosphorus/polyetrafluoroethylene.
 4. A process as claimed inclaim 1, wherein the metal/polymer dispersion layer has a polymercontent of from 15 to 25% by volume.
 5. A process as claimed in claim 1,wherein the metal/polymer dispersion layer has spherical polymerparticles having mean particle diameter of from 0.1 to 0.3 μm.
 6. A heattransfer device for exchange of heat fluids containing a heat transfersurface, a metal/phosphorus layer with a thickness of from 1 to 5 μmbeing applied to said heat transfer surface, a metal/polymer dispersionlayer, in which the polymer is halogenated, being applied onto saidmetal/phosphorus layer and a polymer content from 5 to 30% by volumewithin said metal/polymer dispersion layer.
 7. A heat transfer device asclaimed in claim 6, wherein the metal/phosphorus allay of themetal/polymer dispersion layer and of the metal/phosphorus layer isnickel/phosphorus or copper/phosphorus.
 8. A heat transfer device asclaimed in claim 6, wherein the metal/polymer dispersion layer is adispersion layer of nickel/phosphorus/polytetrafluoroethylene.
 9. A heattransfer device as claimed in claim 6, wherein the metal/polymerdispersion layer has a polymer content of from 15 to 25% by volume. 10.A heat transfer device as claimed in claim 6, wherein the metal/polymerdispersion layer has spherical polymer particles having a mean particlediameter of from 0.1 to 0.3 μm.
 11. A process for reducing or preventingfouling of a surface, comprising coating the surface by electrolesschemical deposition, first with a metal/phosphorous layer, andsubsequently with a metal/polymer dispersion layer, in which the polymeris halogenated.
 12. A process as claimed in claim 2, wherein themetal/phosphorus alloy of the metal/polymer dispersion layer and of themetal/phosporous layer is nickel/phosphorus.
 13. A process as claimed inclaim 4, wherein the metal/polymer dispersion layer has a polymercontent of from 19 to 21% by volume.
 14. A heat transfer device asclaimed in claim 7, wherein the metal/phosphorus allay of themetal/polymer dispersion layer and of the metal/phosphorus layer isnickel/phosphorus.
 15. A heat transfer device as claimed in claim 9,wherein the metal/polymer dispersion layer has a polymer content of from19 to 21% by volume.